For those who like the effects of drafting in cycling backed up by Mythbusters, here’s a comparison between riding a mountain bike at 20 mph solo and on the tail end of a semi. #
Tag: fluid dynamics
Tour de France Physics: Lead-Out Trains
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One of the most impressive cycling techniques for drag reduction on a rider is the lead-out train that delivers a sprinter to the finish line. No current team is better at this than HTC-Highroad. Watch for them in the white and yellow from about ~4:00 in the above video.
The lead-out train begins 5 km or so before the line, with the entire team in a line at the front of the peloton with the sprinter in the final position. The rider at the front will ride for as long and hard as he can, ensuring that the pace is such that no riders from the main field are able to pull ahead. This accelerates the sprinter to higher speeds while sheltering him in the wake of the rest of the team.
One by one, the riders of the team will do their time at the front, expending their energy while protecting the sprinter. The final lead-out rider will be sprinting a few hundred meters from the finishing line; at this point the sprinter in the back may be riding 70 kph while enjoying protection from the wind. Finally, with the finish line in sight, he will swing out around his lead-out man and go all out for the line. Sprinters can hit speeds of nearly 80 kph in these short bursts.
FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See previous posts on drafting in the peloton, and pacelining and echelons.
Tour de France Physics: Breakaways
In cycling, a small group of riders often leave the protection of the peloton in a breakaway. These riders will often spend 80% or more of a stage or race outside of the peloton, trying to reach the finish line before they’re caught. Because the pressure drag is so draining on a lone cyclist, it’s vital that breakaway riders work together. When the wind comes predominantly from the front or back, riders will form one or two lines, riding with their wheels within a foot of one another (see ~0:23). This paceline rotates so that every rider takes a turn at the front, bearing the brunt of the effort while other cyclists recover in their wake, where they experience less drag.
If the wind blows predominantly across the riders, they will form a diagonal line with the frontmost rider rotating behind for shelter from the wind after a pull. This drag reduction technique is called an echelon (see ~1:40). As seen above, for experienced riders the echelon can protect individuals even in bike-stealingly high winds.
FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See part one on drafting in the peloton.
Tour de France Physics: Pelotons
July is well underway and for cycling fans around the world that means it’s time for the Tour de France. This week at FYFD we’re going to do something a little different: in honor of cycling’s biggest race, every post this week will focus on some of the fluid dynamics involved in the sport.
On a bicycle, except when climbing, the majority of a rider’s energy goes toward overcoming aerodynamic drag. Riders wear close-fitting clothes to reduce skin friction and loss to flapping fabric, but most of their drag is pressure-based. A blunt object disturbs the airflow around it, usually resulting in separated flow in its wake. A high pressure region forms in front of the rider and a low pressure region forms in the separated flow behind them. This pressure difference literally pulls the rider backwards. Since drag goes roughly as speed squared, adding a headwind makes matters even worse for a cyclist.
In races, especially on flat stages, the majority of the riders will stay in a large group called a peloton in order to counteract these aerodynamics. By riding in the wakes of those in the front, riders in the peloton experience a much smaller front-to-back pressure difference and thus much less drag. For a rider in the midst of the peloton, the drag reduction can be as great as 40% (#). This allows riders to conserve energy for solo efforts near the end of the race or stage, like breaking away from the peloton in the final kilometers or winning a sprint for the finish line. (Photo credit: Wade Wallace)
STS-135: The Final Shuttle Flight
Condensation clouds form around sections of Atlantis as STS-135–the final space shuttle flight–launches from Cape Canaveral this morning. These clouds, also called Prandtl-Glauert singularities or vapor cones, form at transonic speeds when air accelerates around the vehicle. The area just behind these shock waves experiences a drop in pressure and temperature that brings a localized portion of the flow below the dew point. Rapid condensation of the moisture in the air results. Miss the launch? Watch it here.
High Hopes
This gorgeous high-speed video captures bubbles, droplets, wakes, cavitation, coalescence, jets, and lots of surface tension at 7000 fps. The authors unfortunately haven’t indicated whether this is air in water or something more viscous, but regardless there are some great phenomena on display here. # (via Gizmodo)
Wind Tunnel Testing
A scale model of the Space Shuttle attached to its modified 747 carrier hangs in a NASA wind tunnel. Wind tunnel tests can be used for flow visualization, lift and drag measurements, control system checks and so forth, but mounting models correctly and safely in the tunnel is crucial. Many models use sting mounts that project forward, as this one does, in order to expose the model to freestream flow unimpeded by the mounting mechanism. Any mounts and models must also be sturdy enough that all or part of them does not break off mid-test and fly into the wind tunnel’s fans. #
High-Res Rayleigh-Taylor Instability
When a heavy fluid sits atop a lighter fluid, the interface between the two breaks down through the Rayleigh-Taylor instability. This computation of a 2D interface shows the near fractal behavior of this instability as whorls and eddies of all different scales form and mix the fluids. (submitted by @markjstock)
Whipping Instability
A droplet of glycerol coalescing in silicone oil while subjected to strong electric fields exhibits a whip-like instability reminiscent of fireworks. Check out videos of the phenomenon or see the paper for more information. Happy Independence Day to our American readers!
For more fun, holiday-themed high-speed video, check out PopSci’s fireworks videos.
Triggering Avalanches
Humans often trigger avalanches purposefully before natural ones can occur. Either way, avalanches begin when external stresses on the snow pack exceed the strength within the snow pack or at the contact between the snow and the ground. Acceleration of the snow is gravity-driven. If the snow mixes with air, powder clouds can form that carry snow even further than the main slab. Although the snow itself is not a fluid, once an avalanche gets moving, its behavior can be better modeled as a fluid than as a solid.
